1. Field of the Invention
The present invention relates to high pressure pumps which operate above 10,000 p.s.i. of the type having a reciprocating plunger or shaft portion.
2. Description of the Prior Art
Pumps for pumping liquids and gases under high pressures typically have one or more cylinders, each cylinder having a reciprocating piston or plunger within. Usually a piston refers to a body which moves within a cylinder having seals or rings which are attached to and travel with the piston. A plunger usually moves relative to a seal attached to a cylinder. Normally a closure having a port therethrough is attached to the distal end of the cylinder. Leakage of fluid around the piston or plunger can typically be prevented by various seals and packings. The seals range from simple O-rings to multiple rings of various materials such as elastomers, polymers, rubber, reinforced combinations thereof, brass and teflon. Such a complex seal is disclosed, for example, in U.S. Pat. No. 2,991,003 to R. S. Peterson.
Very often seals are positioned between the moving plunger and the cylinder wall. Consequently, there is relative movement between the seal and the plunger. The art has recognized that if known types of plunger sealing rings are constructed of a soft enough material to provide an effective seal, the seals rapidly become worn and lose their efficiency. On the other hand, if the seals are constructed of a relatively hard material to withstand wear they generally do not provide a good seal. In addition, hard seals may cause scratching or galling of the surfaces which slide past the seal. As the pressure of the pumped fluid is increased the problem of seals is magnified. As a result, in many pumps the pressure is limited by the effectiveness of the seals.
The art has attempted to reduce the wearing and galling problems associated with harder seals by increasing the hardness of the cylinder or plunger surfaces. This can be done by various surface hardening treatments, coating the wall with carbide or using higher strength alloys. All of these techniques increase the cost of the pump and make manufacturing more difficult. Carbide coatings are also brittle and are intolerant of high tensile stress. Furthermore, in some pumps the corrosiveness of the working fluid may limit the choice of alloys. Consequently, there is a need for a relatively low cost sealing system for high pressure pumps particularly for those pumps which operate at pressures above 10,000 p.s.i. There is also a need for a sealing system which is not dependent upon the metals chosen for the cylinder.
Professor Bridgeman in his book The Physics of High Pressure, McMillan Publishing Company, N.Y., 1931, proposed a seal which relied upon a principle of unsupported area sometimes called Bridgeman's principle. Bridgeman states that in order to effect a seal, the hydrostatic pressure in the packing must be maintained at a pressure higher than pressure to be sealed. Prior to the present invention, the art has made only limited application of Bridgeman's principle to piston designs particularly high pressure pumps. For example, in U.S. Pat. No. 4,382,750 to Robertson et al. a free piston compressor for gases and liquids is disclosed in which the positive pressure of the incoming fluid holds a free piston against the piston rod during the intake stroke. Leakage of hydraulic fluid around the piston is prevented by a packing and associated bushing. Although the operation of this bushing and packing are not explained in the specification, one can see from the drawings that Bridgeman's principle is applicable to this system.
The art has commonly relied upon mechanically loaded packings, such as Amagat's fully enclosed packing for high pressure pumps. In a mechanically loaded design, the axial load is supplied by a preloaded gland or flange and is not increased by increasing operating pressure. Under such conditions, the packing wears at much the same rate at low pressure as it does under high pressure. This occurs because the packing pressure does not depend upon the operating pressure. Likewise, operating pressure is limited to something less than the preload pressure of the packing. Consequently, operations at high pressure, 60,000 p.s.i. for example, in systems that have mechanically loaded seals require seals having tremendous preloads. For that reason, there is need for a high pressure pump which does not require a highly preloaded packing and which can operate at pressures of 60,000 p.s.i. and higher.